The sheet-metal working machine can be, for example, a sheet-metal press, by means of which sheet-metal pieces are to be shaped.
There are already known sheet-metal-working machines, on which the shaping of metal sheets is carried out automatically and in preprogrammed manner by making the metal sheets pass automatically through the various operations required for the intended profiling. The manipulating devices used in conjunction with such sheet-metal-working machines are advantageous, particularly as regards the processing of large heavy metal sheets which a single worker, without an aid, can no longer move or can move only with difficulty. To grasp the sheet-metal pieces, there are usually two pairs of rotatable tongs which take hold of the metal sheets laterally, care being taken to ensure that the holding tongs are mounted so as to be displaceable in three dimensions.
In one known arrangement, a gripping device which grasps the sheet-metal piece on opposite sides by means of two pairs of tongs is used. However, for repeated use, there is a disadvantage in employing such tongs, because, when the tongs are used repeatedly, the highly sensitive surface of the sheet-metal piece is damaged, the tongs leaving impressions and other traces behind.
A further known arrangement which has suction cups as gripping members is also unsatisfactory. The use of suction cups has the disadvantage that the sheet-metal piece, particularly when it is thin, has a pronounced sag, so that it is possible to work efficiently only with a large number of suction cups which have to be distributed uniformly along the entire surface of the metal sheet. Since sheet-metal pieces of widely varying sizes are worked, it is not possible to obtain a practical design of a holding device with suction cups.
Finally, the use of clamping strips which are provided in pairs along two parallel edges of the sheet-metal piece. However, such clamping strips can only be used when the sheet-metal pieces have a certain rigidity and do not sag when gripped between the clamping strips.